This invention relates to a process for producing higher molecular weight polyesters.
High molecular weight, crystalline polyesters such as PET (polyethylene terephthalate), PBT (polybutylene terephthalate), PPT (polypropylene terephthalate) have found broad commercial utility including films, fibers, injection molded parts and many others. A wide range of technologies have been developed to achieve the high molecular weight needed for strength properties of these materials.
For Example, PET may be prepared by the direct esterification of terephthalic acid with ethylene glycol. Typically, an excess of ethylene glycol is utilized to react with all the terephthalic acid groups at high temperature. A high vacuum is subsequently applied and transesterification catalyzed for the removal of volatile ethylene glycol and the building of high molecular weight in the molten polymer.
Another method involves the use of dimethyl terephthalate (diester) which is transesterified by reaction with ethylene glycol at high temperature. Once the methanol has been removed, a high vacuum is applied and transesterification catalyzed for the removal of volatile ethylene glycol and the building of high molecular weight.
It is understood by those skilled in the art, that removal of the volatile diol component from the polymer melt during transesterification leads to high molecular weight. High molecular weight may be obtained faster if higher vacuum is utilized. Typically, a high vacuum of less than 5 mm mercury is utilized. Vacuums below 1 mm of mercury are preferred. It is also known that as the melt viscosity increases due to increased molecular weight, the removal of diol becomes more difficult. The increase in molecular weight can become diffusion dependent because of the high viscosity of the molten polyester. This means that the released volatile diol from the transesterification reaction reacts back into the polymer before it can diffuse out of the melt, and be removed. Renewing the surface of the melt can facilitate the loss of diol and increase molecular weight.
Because of the high melt temperature of PET and the above issues, fluidized bed technology for achieving high molecular weight has also been utilized. A PET polymer is prepared using the melt processes described above to produce mid to high molecular weight. The polymer is formed into particles and fluidized in a hot stream of inert gas. The hot gas stream helps remove volatile ethylene glycol and increase molecular weight.